I have to admit that I have never been a fan of The Riddler. I know I've mentioned this on the The Dark Knight thread, but it's buried so deep in there it'd be hard to find. I do like the tattoo idea and a more Zodiac type of killer, but then again, I feel kind of like that's a repeat of the Joker. I know it's not exactly the same, but Joker left clues as to who was going to die, like with the card and the name badges. Can the Riddler really be written as a completely different character? One that won't inevitably be compared to Ledger's Joker?
Dent was my favorite part of The Dark Knight. I absolutely loved his character, loved his whole story arch. That being said, I don't want to see him again in the third. I feel that his character was perfect as is. His revenge was done. Short of spending an entire movie tormenting Gordon and his family, I don't see what else he could do. And the plastic surgery idea is interesting, but I don't see it working. Gordon had a funeral for the guy! Imagine him coming back looking exactly the same. How do people explain that? Also, after surgery, can he really be Two-Face? I know it was his nickname, but still, it doesn't work when he doesn't have the two faces. Ultimately I think it'll disappoint.
I may be in the minority, but I think Batman needs a love interest. She is as crucial to the plot as a villain in my opinion, because she's something that makes Batman weaker, more vulnerable (as this filmed proved). Obviously I don't think an introduction of some random girl is a good idea. It might be viewed as cheapening the image of Rachel. She's not just a throwaway character. I mean, Batman wanted to give up the suit for her. Dent went crazy after her death. She's important, but we don't need another good girl. A Vicki Vale to make to her first appearance and suddenly take away all the heartache Bruce has felt. I think this film needs a bad girl. I don't know who, but I do think it is necessary. I think Catwoman works best. She's well known, she and Batman have a lot of sexual tension, and there's the added bonus of secret identities, reiterating (or possibly just repeating) the theme that Joker puts out there in The Dark Knight: "You see to them, you're just a freak...like me!" Selina Kyle and Bruce could share some scenes, and if done like the Animated Series, it could work well. But as Attila quoted, could Catwoman/Selina be done any better than she was done in Batman Returns? Is there anywhere Nolan can take her character? But then I'm not sold on any of the other female options either, like Talia or Poison Ivy.
So it seems in this long winded post that I haven't actually said anything other than ask more questions. Oh well.